A stack of fuel cells generally has a laminate structure of multiple unit cells. One cooling plate is disposed between each pair of adjoining sub-stacks, each sub-stack consisting of plural unit cells, to cool the stack (unit cells). A flow path of a coolant is formed in the cooling plate, and the flow of the coolant through the coolant flow path cools down the stack. The coolant for the fuel cells is circulated in the stack that carries out power generation, that is, between each pair of adjoining sub-stacks. In order to prevent a decrease in power generation efficiency (that is, to reduce energy loss) due to the leak to the outside of the stack and the resistance of the coolant, the coolant is required to have high insulation performance. The prior art technique applies pure water for the coolant, in order to satisfy the requirements of ensuring the sufficient insulation performance and the sufficient cooling efficiency. The coolant for the stack of fuel cells is further required to have rust resistance, with a view to extending the life of the cooling plates. The general countermeasure to meet this requirement applies stainless steel material having high rust resistance for the cooling plates. Another proposed technique adds iron ions to the coolant as discussed in JAPANESE PATENT LAID-OPEN GAZETTE No. 2-21572.
Such proposed techniques have effects on the stationary, installed medium-sized or large-sized fuel cells and the continuous-driving fuel cells, but do not have sufficient effects on the portable small-sized fuel cells and the intermittent-driving fuel cells, such as fuel cells mounted on the vehicle.
In the case of the intermittent-driving, portable fuel cells, the coolant in the non-working state is cooled down to the environmental temperature. The coolant is accordingly required to have unfreezing performance under the condition that the environmental temperature is below the freezing point. Freezing the coolant may damage a cooling circuit including the cooling plates. The damaged cooling circuit may lead to insufficient performances of the fuel cells.
In order to ensure the unfreezing performance, a coolant for cooling an internal combustion engine may be used as the unfreezing coolant. The coolant for cooling the internal combustion engine is, however, intrinsically used in the parts with no power generation and is not required to have low electric conductivity. Namely such a coolant has extremely high electric conductivity. The electric current flows through a cooling pipe in the stack of fuel cells. The high electric conductivity of the coolant accordingly causes the power generated by the fuel cells to flow into the coolant. This leads to an undesirable power loss. The coolant for cooling the internal combustion engine is accordingly unsuitable as the coolant for cooling the stack of fuel cells.
In the case of the portable fuel cells mounted on the vehicle, reduction in total weight of a fuel cells system including the cooling circuit is an important issue. For the purpose of reduction in weight, it is expected to use a light metal having high heat conductivity, such as aluminum material, for the cooling plates and a heat exchanger. The light metal, however, generally does not have so high rust resistance as that of the stainless steel material, so that the coolant itself is required to have rust resistance.
The object of the present invention is thus to solve the problems of the prior art techniques discussed above and to provide a coolant for a stack of fuel cells having low electric conductivity, rust-preventing ability, high transmission ability, and unfreezing performance.